This invention relates to direct-positive photographic silver halide emulsions and more particularly it relates to direct-positive photographic silver halide emulsions which are spectrally sensitized with novel dyes containing 5,6-dihydropyrrolo[2,1-a]isoquinoline ring.
Usually, a negative image is obtained by subjecting a photosensitive silver halide material to a suitable exposure and then to a development. Furthermore, there has also been known the reversal phenomenon according to which a positive image is obtained by exposing a certain photosensitive silver halide material and then developing thus exposed photosensitive material. This reversal phenomenon includes Herschel effect, Villard effect, solarization, etc., among which the solarization has been used for the direct-positive photosensitive silver halide materials of high speed. The solarization is such a phenomenon that when a photosensitive silver halide material is exposed to light including its sensitive region and then is developed, the optical density which increases with exposure in the optimum region begins to decrease with exposure in the overexposure region. This solarization is also observed in photosensitive silver halide materials which have been fogged with light or a chemical fogging agent to an appropriate extent and a direct-positive image can be obtained.
According to this invention, it is intended to obtain high-speed direct-positive photographic silver halide emulsions effectively spectrally sensitized in the blue, or green, or red region by novel spectral-sensitizing dyes containing 5,6-dihydropyrrolo[2,1-a]isoquinoline ring.
Many dyes have been known as spectral sensitizers for the usual negative emulsions. However, use of these dyes as spectral sensitizer for direct-positive silver halide emulsions often results in various problems, e.g. low contrast (flattening) of characteristic curves, re-reversal (a phenomenon owing to which the optical density increases again with exposure over the solarization region), etc.
Furthermore, many of the dyes known as spectral sensitizers for direct-positive silver halide emulsions, for example, those disclosed in U.S. Pat. No. 3,431,111, have the defect that they cause color stain based on the dye which remains even after processing of photosensitive materials.